"And I thought my jokes were bad!" sneers Heath Ledger's Joker in The Dark Knight. The character is today at the centre of a row about dodgy humour, but it's got nothing to do with his gags. A poster has been appearing across Los Angeles and other parts of the United States in which President Barack Obama is depicted as the supervillain, with the word "socialism" written beneath his made-up face. Who has been pasting it up is not widely known but it marks the first time a negative representation of Obama has gained mainstream traction, as well as the American right's first successful use of street art. But is it dangerous? And is it any good?

Street art is traditionally a tool of the disenfranchised and outlawed, and it was this valence that made its use by Obama's supporters in last year's election – most famously Shepard Fairey's Hope image – so potent. Today, justifiably or not, many of Obama's opponents see themselves as disenfranchised outsiders. Laying claim to this mode of expression is a clever way of expressing that sentiment, rhetorically marking the new administration as "power" or even "tyranny", and its opponents as rebels. The choice of medium is smart. The message … not so much.

In fact, there was probably only one image that gained a greater hold on American visual pop culture last year than Ledger's Joker: Fairey's Obama. No other politician in recent history – let alone a mere candidate – has had their iconic status established as quickly and solidly as Obama's was in 2008. For this reason alone, the fact that his look and the Joker's have been conflated is less surprising than the fact that it didn't happen sooner. Actually, thanks to Australian artist James Lillis, it did, albeit from the other direction. And the "new" poster is in fact quite old. First spotted as early as April, the poster is apparently based on an image by Chicago student Firas Khateeb, uploaded to Flickr before Obama was even inaugurated. Only now, with Obama's healthcare plans becoming a focal point for opposition, has it caught on.

Nor is the idea of the Joker-in-Chief new. As the Newsbusters site notes, Vanity Fair ran an image last year of then-president George W Bush, made over à la Ledger without provoking anything like the stir the new poster has caused. Those crying foul now include president of the Los Angeles Urban Policy Roundtable, Earl Ofari Hutchinson. "Depicting the president as demonic and a socialist goes beyond political spoofery," Hutchinson reportedly said. "It is mean-spirited and dangerous."

Socialist, of course, is as grave a slur as can be directed against an American politician. But isn't any sitting president, even a broadly popular one, a legitimate target for satire? It's surely an irreducible facet of freedom of expression that leaders be held up to ridicule. Unless that ridicule includes incitement to violence, objections generally boil down to matters of taste. The Joker poster is mean-spirited, sure, as plenty of satirical images are, but is it dangerous? If it is, it comes down to race. As with the New York Post cartoon that conflated an Obama initiative and the shooting of a rogue ape, it's a moot point whether the intention was bigoted: the simple fact of publishing an image of a black president with his face covered in white greasepaint but for the eyes and a red slash of a mouth can't help but evoke a minstrel aesthetic, even if it's in reverse. An argument could be made that the inflammatory potential of such imagery is likely to outweigh any potential satirical value. This, presumably, is what Hutchinson means by "dangerous".

Racial insensitivity aside, a more fundamental objection to the poster is that it is not very funny. Successful satire draws attention to aspects of a politician's persona that are recognisably there: George Bush-as-cowboy was a connection acknowledged by both his supporters and opponents, even if they disagreed about the implications. The connections between the Joker and Obama are far more obtuse – the president's measured tone hardly makes him an obvious candidate for agent of chaos. In any case the character's anarchist charisma is quite at odds with the charge of Big Brother authoritarianism the poster levels at Obama. Should the image gain any continued traction, then, it will be as a blunt locus of demonisation. For the time being, though, it seems that more Americans see their leader as the good guy.